Instructions on how to customize the skins are here: http://it.cmsaspnet.com/?pn=32&ar=64 (italian only). Can anyone help me to convertirele in English?
To change some features of the skin (colors fonts, etc), you can also use the appropriate plugin: Setup > Skin customization > Setup button (in new windows you find the plugin's setting), save the plugin's setting and active this plugin and save te setup.